The present invention relates to a phase modulation system, and more particularly to a phase modulation system suitable for subscriber transmission.
In base band digital transmission by way of a metallic medium, codes that can be regarded as phase modulation signals, such as the biphase codes (also known as the Walsh codes), are used by the prior art. Such phase modulation codes include quadriphase phase modulation (QPh) codes, which are also called the complex Walsh codes.
The QPh codes, however, have a wide transmission signal band because of their use of square waves and consequently cause considerable obstruction of other systems in crosstalk. Therefore, by the prior art the square waveform is band-limited with a high-order low-pass filter, which thus complicates the circuitry and thereby creates a new problem.